1. Field of the Invention
The present invention relates to an arrangement for assisting in delivering an up jar blow by a jarring mechanism in a well string to unstick a fish, or stuck element, in a well bore.
2. Description of the Prior Art
The most pertinent prior patents with which Applicants are familiar are U.S. Pat. Nos. 2,265,431 issued to R. L. Kerr on Dec. 9, 1941 and 2,801,078 issued to W. L. Medders et al on July 30, 1957. Kerr and Medders each disclose an independent piston arrangement which is connected with a hydraulic jar and which piston arrangement may be moved to compress gas in a chamber as the jar mechanism is actuated to enable a pull force to be developed in a well string. However, only the compressed gas in each patent is utilized to unlock the jar means whereby the piston in response to the compressed gas may then move upwardly and deliver a jarring blow to the well string. It can be appreciated that some of the energy of the compressed gas in each of the above referenced patents is utilized to actuate and effect release of the jar mechanism, whereas the instant invention is constructed and arranged so that the jar mechanism is externally controlled in that the well string is directly connected mechanically with the jar mechanism. Thus, the jar mechanism is engaged by manipulation of the well string so that a pull force may then be developed in the well string by manipulation thereof to build up energy in the well string which is utilized for applying a jar blow when the jar mechanism releases in response to the pull force. Thus, the mass in the Medders et al and Kerr Patents that is involved in the impact to the well string is confined to the internal structure of the tool. On the other hand, in the instant invention, the mass involved in the impact is not confined to the internal structure of the tool, but is external and can be varied. Accordingly, in each prior patent if the mass is constant and the pressure of the charge gas is constant, then the impact is constant; whereas, in the instant invention, the mass may be varied and even with a constant gas pressure, the impact can be varied externally of the tool at the selection or control of the operator.
Also, in the Medders et al structure as well as in the Kerr patent, varying the mass above the tool can have no effect in increasing the energy employed in the impact. On the other hand, increasing the mass above the tool in the prior art can detract or diminish the impact blow; whereas, in the present invention, an increase in mass above the tool increases the impact blow. Neither patent discloses an arrangement to neutralize the hydrostatic head in the well bore acting on the jar mechanism or means to neutralize the hydrostatic head of the well bore acting on the compressed gas chamber.
Further, each preferably provides a piston arrangement which sealably engages with a cylinder so that movement of the piston in response to the gas pressure must overcome such frictional engagement.